The present invention relates to an apparatus for the subjective and objective determination of refraction of the human eye.
Objective-type eye refractometers are known, for instance from "ABC der Optik" (ABC of Optics) 1961, pages 742/743, in which a test object is focused on the retina of the eye to be examined and the light reflected by the eye is observed. An optical element arranged in the illumination ray path, for instance a lens or a prism, is displaced until the test image appears sharply on the retina, the position of this optical element then being a measure of the refraction value sought.
These known apparatuses have the disadvantage that they do not permit simultaneous subjective determination of the refraction, and that only a monocular determination is possible.
German Federal Republic Pat. No. 1,299,907 discloses an apparatus for the subjective and simultaneous objective determination of refraction in which two test marks are projected simultaneously in different spectral regions on a screen and the test subject observes said screen through a phoropter.
The light reflected by the retina of the eye is reflected out of the illumination ray path via a partially transmitting mirror and focused by means of an optical system in an image plane. The observer varies the setting of the phoropter until the image of the retina in said image plane is sharp. The patient can note the improvement in vision by observing one of the test marks and can thus himself participate in the optimal adjustment.
This apparatus has the disadvantage that correction is possible only in discontinuous steps and only monocularly, and that the known instrument myopia occurs, due to the phoropter which is arranged in front of the eye of the patient.
From U.S. Pat. No. 3,927,933 there is known an apparatus for the subjective and objective determination of refraction which is developed as so-called free-view apparatus. In this apparatus, a test mark is focused, via projection ray paths which are spaced from each other, onto a concave mirror which is arranged at a predetermined distance from the test subject and is viewed by the latter with both eyes. In both of the projection ray paths there are provided adjustment systems of continuously variable spherical and astigmatic action which are displaced until the test subject sees the test mark sharply. The light reflected by the eyes of the test subject is reflected-out, after reflection on the concave mirror, via a partially transmitting mirror into two observation ray paths which contain the same adjustment systems as the projection ray paths.
This apparatus has the disadvantage that the concave mirror must be relatively large so that the pupil of the eye of the test subject can adjust itself to the test image and that, as a result, the entire instrument is of very large size. Moreover, as a result of the large distance between the eyes of the test subject and the concave mirror, the displacement paths of the adjustment systems are large, and this also contributes to the large size of the instrument.